This invention relates to an apparatus and process for vaporizing liquefied natural gas, heated to ambient temperature suitable for use, for example to a temperature of 50xc2x0 F. to 60xc2x0 F.
The evaporators presently in use in the USA are predominantly of the submerged combustion type, elsewhere in the world other types like open rack sea water type and intermediate fluid type are in use.
Evaporators of the submerged combustion type comprise a water bath in which the flue gas tube of a gas burner is installed as well as the exchanger tube bundle for the vaporization of the liquefied natural gas. The gas burner discharges the combustion flue gases into the water bath, which heat the water and provide the heat for the vaporization of the liquefied natural gas. The liquefied natural gas flows through the tube bundle. Evaporators of this type are reliable and of compact size, but they involve the use of fuel gas and thus are expensive to operate.
Open rack type evaporators use sea water as a heat source for the vaporization of liquefied natural gas. These evaporators use once-through seawater flow on the outside of a heat exchanger as the source of heat for the vaporization. They do not block up from freezing water, are easy to operate and maintain, but they are expensive to build. They are widely used in Japan. Their use in the USA and Europe is limited and economically difficult to justify for several reasons. First the present permitting environment does not allow returning the seawater to the sea at a very cold temperature because of environmental concerns for marine life. The present permitting environment allows only a small decrease in temperature before returning the seawater back to the sea, which would require a very large sea water quantity to be pumped through the system, if the terminal vaporization capacity was designed for a commercial size as economics would require. Also coastal waters like those of the southern USA are often not clean and contain a lot of suspended solids, which could require filtration. In addition the sea water intake structure would have to be located far away from the evaporators in most cases because of location restraints or to get to deep and clean sea water at the intake. With these restraints the use of open rack type vaporizers in the USA is environmentally and economically not feasible.
Evaporators of the intermediate fluid type use a refrigerant like Freon or Propane having a low temperature of solidification to transfer the heat from a warm water stream to the liquefied natural gas. This is achieved by heating the liquid refrigerant in a reboiler type exchanger with ambient once-through water in the tube bundle. The refrigerant vaporizes, condenses to liquid on the cold liquefied natural gas exchanger tubes located in the vapor space of the exchanger and falls back into the liquid refrigerant bath, where it is again vaporized. The heat of condensation of the refrigerant provides the heat of vaporization of the liquefied natural gas. These type vaporizers are less expensive to build, but have the same permitting restraints in the USA as the open rack types.
The main objective of this invention is to provide an apparatus and process for vaporizing liquefied natural gas, which utilizes ambient air to provide the heat for the vaporization process.
A further objective is to provide an apparatus and process to heat cooled water from a heat exchanger by ambient air for return to the heat exchanger.
An additional object of the invention is to provide a supplemental heater for heating the cooled water from the heat exchanger.
The above objects accomplish the extraction of heat from the environment for the vaporization of liquefied natural gas in large quantities and with the least effect on the environment and on marine and terrestrial life.